CAN

CAN#

Controller Area Network. A differential pair plus ground; message-based, multi-master, no addressing of nodes but addressing of messages. The protocol on every modern car’s OBD-II port, on industrial machines, and on robotics platforms. Reverse-engineer a car by capturing the CAN bus, isolating frames that change when the operator presses a button, and replaying them.

Wires#

Signal

Direction

Purpose

CANH

Bidirectional

High line of the differential pair; ~3.5 V dominant, ~2.5 V recessive.

CANL

Bidirectional

Low line; ~1.5 V dominant, ~2.5 V recessive.

GND

Reference

Ground; not always run on short buses but recommended.

Each end of the bus terminates with a 120 Ω resistor between CANH and CANL.

Wire format#

Logic is dominant (0) and recessive (1); a dominant bit wins on the wire, which is how multiple transmitters arbitrate without a master. Frames carry an 11-bit (CAN 2.0A) or 29-bit (CAN 2.0B) identifier, up to 8 bytes of data (64 bytes in CAN FD), and a CRC.

Common speeds are 125 kbps (low-speed bodies, comfort buses), 500 kbps (high-speed powertrain), and 1 Mbps. CAN FD raises the data-phase bit rate to 5 Mbps or more.

  • SOF (1 bit), start of frame.

  • Identifier (11 or 29 bits), the message ID. Lower IDs win arbitration.

  • RTR / IDE / r0, control bits.

  • DLC (4 bits), data length code.

  • Data (0–8 bytes, or up to 64 in CAN FD).

  • CRC (15 bits) + delimiter.

  • ACK slot, the receivers’ acknowledgement.

  • EOF, end of frame.

Pads#

The OBD-II 16-pin J1962 connector is the operator’s most common entry point.

Pin

Signal

4

Chassis ground

5

Signal ground

6

CAN H (high-speed CAN, 500 kbps)

14

CAN L

16

Battery + (12 V)

DB9 pinouts vary; the operator confirms with a meter before hooking up a transceiver.

Tools#

Tool

Effect

can-utils (candump, cansend, cansniffer, cangen)

Linux SocketCAN userspace; capture, replay, fuzz.

Linux SocketCAN (can0, vcan0)

In-kernel CAN stack; ip link brings a CAN interface up like an Ethernet adapter.

CANalyzer, PCAN-View

Vendor capture and replay tools.

CANtact, Macchina M2, USB2CAN

USB-to-CAN bridges; Macchina M2 is the operator’s OBD-II workhorse.

Wireshark

Decode CAN captures (.log, .pcap) with the CAN dissector.

References#

  • ISO 11898 (CAN physical and data link layers).

  • SAE J1939 (heavy-duty vehicle networks) and SAE J1979 (OBD-II diagnostics).

  • can-utils.

  • RS-485 for the differential industrial peer.

  • UART for the async serial neighbor.

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